LiveUpdate 5.1.2 is not compatible with NAV 10.1.2.   Sorry that was not made clearer.  Since NAV 10.1.2 is no longer a supported product, our recommendation is to update to NAV 11.

In the meantime, you can download Intelligent Updaters for the virus definitions from here:

http://www.symantec.com/business/security_response/definitions/download/detail.jsp?gid=nmc

Use the NAVM9_Installer for NAV 10.x.